1. Field of the Invention
This invention relates to circuit protection devices suitable for surface mounting onto a substrate, and to assemblies of such devices.
2. Introduction to the Invention
Circuit protection devices which exhibit positive temperature coefficient of resistance (PTC) behavior are well known. Due to this PTC behavior the resistance increases anomalously from a low resistance, low temperature state to a high resistance, high temperature state at a particular temperature, i.e. the switching temperature Ts. Under normal operating conditions, a circuit protection device which is placed in series with a load in an electrical circuit has a relatively low resistance and low temperature. If, however, a fault occurs, e.g. due to excessive current in the circuit or a condition which induces excessive heat generation within the device, the device “trips”, i.e. is converted to its high resistance, high temperature state. As a result, the current in the circuit is dramatically reduced and other components are protected. When the fault condition and the power are removed, the device resets, i.e. returns to its low resistance, low temperature condition. Fault conditions may be the result of a short circuit, the introduction of additional power to the circuit, power surges, or overheating of the device by an external heat source, among other reasons.
Preferred circuit protection devices comprise a conductive polymer composition, i.e. a composition comprising a polymeric component, and, dispersed in that component, a particulate conductive filler. Preferably the polymeric component comprises a crystalline polymer. During the tripping event the device expands as the crystallites in the polymer melt. It is known that for optimum performance, expansion of the polymer should not be significantly restricted or otherwise the PTC anomaly will be diminished. Therefore, attachment of electrodes, e.g. metal foils, to a PTC element must be carefully done. Additional restriction may occur if the mounting of the device, e.g. onto or through a substrate such as a printed circuit board, results in a restraining force either when the polymer expands or contracts. In order to avoid substantial restriction, conventional radial leaded devices, in which wire leads are attached to metal foil electrodes sandwiching a PTC element, often contain “kinks” or other non-rigid sections as part of the leads. This means that, although the end of the lead that is in contact with the substrate may be rigidly restrained, the non-rigid section allows expansion and contraction to occur. See, for example, U.S. Pat. No. 4,685,025 (Carlomagno), the disclosure of which is incorporated herein by reference.
Surface mount devices, which are attached directly to the surface of the substrate, may also suffer from restraining forces. For example, if the device is positioned parallel to the surface of the printed circuit board or substrate and has a configuration in which a top and/or bottom lead (often in the form of a metal “strap”) is soldered or welded onto the substrate, the lead(s) should be positioned so that they do not restrict expansion. This can be achieved by making the top lead sufficiently long or flexible so that any restraining forces are minimized.